This invention pertains to photographic elements and silver halide emulsions comprising a mixture of at least two different dye-forming image couplers, and to methods of developing images using the elements.
Images are commonly obtained in the photographic art by a coupling reaction between the development product of a silver halide developing agent (e.g., an oxidized aromatic primary amino developing agent) and a color-forming compound known as a coupler. The dyes produced by the coupling reaction are indoaniline, azomethine, indamine or indophenol dyes, depending on the chemical composition of the coupler and the developing agent. Ordinarily the subtractive process of color formation is employed, and the resulting image dyes are usually cyan, magenta and yellow dyes which are formed in or adjacent to silver halide layers sensitive to red, green and blue radiation, respectively. Typically, phenol or naphthol couplers are used to form the cyan dye image, pyrazolone or pyrazolotriazole couplers are used to form the magenta dye image, and acylacetaniline couplers are used to form the yellow dye image.
Image coupler blends can be used as aggregates to attain properties intermediate between those of the individual component image couplers. Typically, blends provide levels of fog density (Dmin), gamma, image density formation (which may be quantified as Dmax) and dye hue which vary in a parallel fashion and which can be readily estimated by interpolation from the values associated with each individual coupler, as weighted by the relative quantity of each coupler and by the relative coupling reactivity of each coupler.
Blends of cyan dye-forming couplers have been employed in this fashion to enable improved physical properties such as decreased coupler crystallization during manufacture or storage while maintaining other desired photographic properties. Such a use is described, for example, in U.S. Pat. Nos. 4,842,994; 4,865,959; 4,885,234; and published European Patent Application 0 434 028. Related uses of blends of cyan dye forming image couplers are described in U.S. Pat. No. 5,084,375; published European Patent Application 0 254 151 B; Japanese Kokoku J91/016,102 B; and Japanese Kokai J03/242,644 A.
Blends of magenta dye-forming image couplers that can be used in a single layer of a color paper are known. For example, Japanese Kokai 61-80251 mentions that two magenta image forming couplers of the same hue can be used in the magenta record of a color paper. No criteria for selection of specific magenta image dye-forming couplers to be combined are set forth in this reference, however. Furthermore, neither the properties nor the potential advantages of such combinations are described.
Use of two magenta dye-forming image couplers, each of narrowly specified structure, to provide desirable dye hue while enabling improved formalin resistance is described at U.S. Pat. No. 4,600,688. The density forming properties appear to be just those expected from the aggregation of the individual components while the dye hues and formalin resistance are described as being unexpected based on the individual properties of the components. This patent discloses that the two magenta dye-forming image couplers may be employed as a blend in a single photographic layer or may be employed individually in two or more photographic layers sensitized to substantially the same region of the electromagnetic spectrum. Examples illustrating both usages are provided. The aggregates described appear to have no unexpected impact on image density formation or gamma.
Certain magenta dye-forming image couplers, such as coupler M-8 of U.S. Pat. No. 4,443,536, are highly useful because of the improved dye hue and dye stability, reduced unwanted absorption and improved formalin resistance that they exhibit after color development. For this reason such couplers are often preferred to couplers such as CC-11 of the '536 patent. Coupler M-8 of the '536 patent can, however, exhibit less than fully satisfactory dye density formation after an image exposure and development.
Efforts to improve the dye density formation performance while maintaining the desired dye hue and stability characteristics have led to magenta dye-forming image couplers such as compound V of EP 0 285 274 (corresponding to Romanet et al., U.S. Ser. No. 23,518) and the compound at page 12, line 5 of EP 0 284 240. While these compounds provide improved dye density formation and improved gamma over those of the '536 patent, they also exhibit a higher than desirable degree of fog growth.
One approach to enabling both improved image dye hue and stability and dye density formation involves providing combinations of magenta dye-forming image couplers with chalcogenazolium salts as described in EP 0 359 169 A. The higher than desirable fog growth may, however, persist in this case.
Another approach has been to use alternative coupler solvents which may alter the partitioning of the coupler or the image dye formed from the coupler in the gelatin matrix of the photographic element as described in U.S. Pat. No. 4,808,502. Such alternative solvents, however, can lead to activity changes in the coupler and hue changes in the dye formed from the coupler.
There has thus been a need for photographic elements which display low fog density together with good density in image-forming areas. Such photographic elements should exhibit superior image-to-fog discrimination.